1. Field of the Invention
The present invention relates to a small-sized, portable ultraviolet ray measuring apparatus suitable for a so-called sunburn monitor which can measure, at high accuracy, the light intensity of an ultraviolet band of sunlight and the amount of irradiation obtained by accumulating the intensity as the time proceeds, and can inform a user of sunburn before his or her skin gets inflamed.
2. Description of the Related Art
The skin, when bathed in the sun for a long period of time, is inflamed due to the light (ultraviolet rays) of an ultraviolet band included in the sunlight. The degree of the skin inflammation depends on the amount of irradiated ultraviolet rays. This amount of irradiated ultraviolet rays is defined as a product of the intensity of ultraviolet rays (watt/m.sup.2) and the irradiation time (seconds) and is expressed in the unit of Joul/m.sup.2.
There are various ways to express the amount of irradiated ultraviolet rays; MED (Minimum Erythema Dose) and SPF (Sun Protection Factor) are used as a skin-related index.
Of these indexes, the MED is defined by the minimum amount of irradiated ultraviolet rays required to cause "slight erythema" when ultraviolet rays are irradiated on a plurality of portions on the skin while changing the amount of irradiation. In other words, the MED is a reference for a threshold value to predict the beginning of sunburn.
The MED is said to take specific values for different skins, such as for different races or different skin types; for example, the average MED for Japanese people is said to fall within a range of approximately 40 to 100 kJ/m.sup.2.
The SPF, on the other hand, is an index to indicate how many times greater the MED becomes when a sun screen is coated on the skin. In other words, it is an index to indicate how many times the amount of irradiated ultraviolet rays are prevented by the use of the sun screen, as compared with when no sun screen is used.
Therefore, the SPF and MED have the relation expressed by the following equation: E1 ? ##STR1##
A conventional apparatus for quantitatively measuring the light of an ultraviolet band in the sunlight comprises a silicon photodiode having a spectral sensitivity in the ultraviolet band and an ultraviolet-ray transmitting/visible-ray absorbing type glass filter provided on the light-receiving face of this photodiode for canceling the spectral sensitivity in other bands except the ultraviolet band of the silicon photodiode. However, the glass filter has a sub-transmitting band near the wavelength range of 650 to 1000 nm and the spectral sensitivity of the silicon photodiode extends to the infrared band. This conventional apparatus, therefore, has a spectral sensitivity not only in the ultraviolet band but also in the near infrared band, thus providing a lower measuring accuracy. Since the light transmittance of the ultraviolet-ray transmitting/visible-ray absorbing type glass filter varies when ultraviolet rays are irradiated for a long period of time, the measuring accuracy or sensitivity varies with time.
A solution to the above shortcomings is proposed in Japanese Laid-Open Patent Publication No. 60-178321; in which the silicon photodiode in the aforementioned apparatus is replaced with a gallium-arsenic-phosphorus photodiode having no spectral sensitivity in the wavelength of 680 nm or above. However, the gallium-arsenic-phosphorus photodiode has a very low spectral sensitivity, under 1/3 of that of the silicon photodiode, in the ultraviolet band of the wavelength of 390 nm or below, and its spectral sensitivity would be further lowered by a combination of the gallium-arsenic-phosphorus photodiode with an ultraviolet-ray transmitting/visible-ray absorbing glass filter. The proposed apparatus, therefore, has a lower sensitivity.
Further, there is a demand of users for a function that provides an at-a-glance visual confirmation of the intensity of actually bathing light of the ultraviolet band and the amount of irradiation of already bathed light of the ultraviolet band in the form of, for example, a bar graph to take a proper action or measures to prevent the undesired sunburn, in addition to a numerical display of the light intensity of the ultraviolet band and an accumulated value of the light intensity (i.e., the amount of irradiated ultraviolet rays).
As mentioned earlier, the influence of the light intensity and the accumulated value of the light intensity on sunburn differs depending on the types of skins. More specifically, in a case where two persons, A and B, bathe in the same amount of ultraviolet rays of the same light intensity, A may get severely sunburned while B may not get sunburned so much.
In this respect, some people using this kind of ultraviolet ray measuring apparatus as a sunburn monitor want to know information as to what the value of the light intensity and the accumulated value of the light intensity measured by the apparatus mean to the sunburn state of their own skins. That is, people want to know if, the light intensity of the ultraviolet band and the accumulated value of the light intensity will excessively advance the sunburn and become dangerous to their type of skin. For instance, users want to know how much damage will be caused to their skins if they keep bathing in the bathing light of the ultraviolet band for a prescribed period of time, how many multiples of the standard MED value correspond to that amount of irradiated ultraviolet rays what SPF value the cosmetics they are using should have to prevent the sunburn due to such ultraviolet rays, etc. In other words, when the ultraviolet ray measuring apparatus is used as a sunburn monitor, some people desire that the light intensity to be measured and the accumulated value of the light intensity be directly displayed as the numerals representing how many times greater these values are as compared with the standard MED value for the type of the skin of the user, or that the SPF value be also displayed at the same time in association with the numerals.